1. Field of the Invention
The present invention concerns a linear-type charge transfer photosensitive device.
2. Description of the Prior Art
Linear charge transfer photosensitive devices are well known from the prior art. These devices generally comprise a "strip" constituted by aligned photosensitive elements. The image to be analyzed is projected line after line on the strip; either the strip or the image to be analyzed is displaced. An optical element can also be used that projects line after line the image to be analyzed, immobile, on the immobile strip.
The charges accumulated in the strip are periodically transferred in parallel in a charge transfer shift register, then transferred in series towards the output of the register where they are read; i.e. converted into a voltage that is amplified.
The charge transfer shift register by its dimensions conditions the minimal dimensions that can be given to the photosensitive elements of the strip, and thus the resolution of the device.
Consequently, two shift registers located on either side of the strip are used. Each register processes half of the photosensitive elements and this allows to double the resolution by doubling the number of the addressable photosensitive elements.
The resolution of the photosensitive devices remains, however, limited by the dimensions of the shift registers used.
For example, in the case of a biphase structure transfer charge shift register, in which the dissymetry is ensured by an overthickness of the oxide, each stage of the register that processes a photosensitive element occupies about 20 .mu.m.
If two registers are used, the photosensitive elements have a pitch that cannot be smaller than 10 .mu.m.
Several solutions have been proposed in order to diminish the pitch, among which can be cited:
the utilization of registers in coil, which present above all the drawback of being difficult to design;
doubling the number of registers, i.e. the utilization of four registers instead of two;
the sequential transfer of data from the strip towards the registers, this sequential transfer being controlled by a logic shift register;
furthermore, the introduction of a memory between the photosensitive strip and each register, the memory receiving data that are transferred in two steps into each register.
The problem that occurs is that the known solutions generally allow to double the pitch but substantially double the space occupied by the photosensitive device. The present invention overcomes this problem and allows to double the pitch without increasing the space occupied by the device.